spanner: Cleanup TestSessionHealthCheck
spanner: Cleanup TestSessionHealthCheck

Replaces the 'wait for one second' statement with a
wait-for-condition statement.

Change-Id: I7d0c48c6a52ca0115915d2be8c3ee5ffcec60965
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/47616
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go